Since Si has advantages of technical maturity, low costs, a high mechanical strength and being easily made in a large size, the III-V semiconductor materials grown extendedly on Si, especially GaAs, are very attractive. If the epitaxial growth of GaAs material with a high quality on Si was realized, the production cost of important semiconductor devices such as GaAs solar cells, optoelectronic detectors and so on, would be greatly reduced, and the combination of microelectronics and photoelectrons would also be realized, which have a broad application prospect. However, there exist some problems with the epitaxial GaAs thin films on the Si substrate. On the one hand, the lattice constant of Si is smaller than that of GaAs, and there are more than 4% lattice mismatches between them. This results in a large number of misfit dislocations in GaAs during the growth and deteriorates the device performances. On the other hand, defects of the surface properties of the Si substrate, such as twin crystals, and anti-phase domain, are likely to occur, especially when there are a large mismatches between the epitaxial material and the substrate. The formation of these defects causes the epitaxial film surface to form a large number of pyramid-shaped or flute-type protuberances, and seriously affects the surface flatness of GaAs semiconductor devices.
In order to suppress the effects of dislocation due to the lattice mismatch and twin crystals on the material properties, the most direct approach is to eliminate the stress between the epitaxial film and the substrate. The common method of eliminating the stress is to insert several graded components and thicker buffer layers between the substrate and the epitaxial film. However, the growing steps of this graded structure buffer layer are often cumbersome, and it is difficult to precisely control compositions, thicknesses, and crystal qualities of material of each layer, thus affecting the quality of the GaAs film finally obtained. Therefore, in order to obtain a GaAs film having a low defect density and a high quality, it is necessary to optimize the buffer layer growth process.